1. Field of the Invention
The present invention relates to an Ethernet passive optical network (EPON). More particularly, the present invention relates to a multicast transmission in the Ethernet passive optical network.
2. Description of the Related Art
At present, the goal of standardization of Gigabit Ethernet and MAC technology for an asynchronous transfer mode passive optical network (hereinafter, referred to as ATM-PON) has been already completed, and the contents thereof are described in IEEE 802.3z and ITU-T G.983.1. The first type of PON standardized ATM-PON. In the ATM-PON, upward or downward transmission of frames, each of which consists of a predetermined number of ATM cells, is performed. In a PON having a tree-shaped structure, an Optical Line Termination (hereinafter, referred to as “OLT”) properly inserts downward cells in the transmitted frame, and then the downward cells are distributed to each Optical Network Unit (hereinafter, referred to as “ONU”).
FIG. 1 is a block diagram showing a physical network structure of a conventional passive optical network.
As shown in FIG. 1, the passive optical network includes one OLT 100 and more than one ONUs 110-1 to 110-3 connected to the OLT 100. FIG. 1 shows three ONUs 110-1 to 110-3 are connected to one OLT 100. More than one end user 120-1 to 120-3 (users or network equipment) may be respectively connected to the ONUs 110-1 to 110-3, although the diagram only shows one end user per ONU. Data 131 to 133 transmitted by the users 120-1 to 120-3 are transmitted to the OLT 100 via the ONUs 110.
As shown in FIG. 1, in the structure of the Ethernet Passive Optical Network, which transmits 802.3 Ethernet frames via a point to multi-point network, in the case of upward transmission, all the data of the ONUs are accessed by means of a Time Division Multiplexing (TDM) method. An Optical Distribution Network (ODN), which is a passive element, prevents the data from colliding with each other by means of a ranging method. In other words, in the case of upward transmission, each data of the ONUs 110 are multiplexed and then transmitted to the OLT 100. Also, in the case of downward transmission, each of the ONUs 110 (hereinafter, a plurality of ONUs are referred to as 110-n) having received data broadcast by the OLT 100 selectively receives only data, which the ONU must receive, from among the broadcasted data.
For this purpose, each frame in upward or downward transmission has a field arranged in a dedicated ATM cell or a general ATM cell, by which messages can be sent or received at predetermined intervals. With the development of the Internet technology, subscriber-sides have required more and more bandwidths and have been attracted to an end-to-end transmission by means of Gigabit Ethernet which is relatively low-priced and can secure a higher bandwidth, in comparison to the ATM technology, which requires relatively expensive equipment, has limitation in the bandwidth, and must perform segmentation of IP packets. Thus, even in the PON structure of the subscriber network, the Ethernet type is required rather than the ATM.
At present, EPON standardization has been progressing by IEEE802.3ah EFM (Ethernet in the first mile) TF with the aim of September, 2003. An issue of the standardization is the problem of matching, which relates to a layering between an OAM layer and other layer and detail specification works have progressed.
In addition, in the Draft v1.0 of the EPON standardization having been progressing by the IEEE802.3ah, when a communication is performed between an OLT and an ONU, a logical link ID (hereinafter, referred to as LLID) is inserted into a preamble in order to check each packet about whether or not the destination of the packet is the OLT or ONU itself.
FIG. 2 shows a preamble format including a LLID in an EPON and the preamble format has been described in the IEEE 802.3ah baseline. Referring to FIG. 2, 2 bytes are assigned to the LLID in the preamble. A first one byte SOP in the preamble is a start of packet (SOP) byte 10 and represents that a packet has started. The four bytes after the SOP byte 10 are bytes 20 reserved for future purposes. Further, two bytes 30 after the reserved four bytes 20 are assigned to the LLID. A field 40 after the LLID 30 is a CRC which plays a error-checking role, particularly of a checksum of the preamble, which includes meaningful information, in the EPON, in contrast with an existing preamble which is necessary for a reception side to synchronize. The LLID 30 is two bytes in length includes a mode bit 32 having a size of one bit and an ID part 34 having a size of 15 bits. The mode bit 32 represents that a received packet is one of a broadcast packet or a unicast packet. The ID part 34 is used as an identifier of each transmission or reception party. One LLID may be assigned to each ONU, or the LLID subdivides and then other IDs may be assigned according to each service or a user connected to an ONU. However, it is not determined yet which method to employ from among the above two methods.
Hereinafter, an assignation process of the LLID will be described with reference to FIG. 1. A plurality of ONUs 110, which are powered on in an initialization process of an EPON, must pass through an auto-discovery process in which the ONUs 110 are registered to an OLT 100. Herein, the OLT 100 assigns a particular ID to each MAC address by means of MAC addresses of transmission/reception parties, which require registration, and then creates/manages an ID list in a table of the OLT 100. After the registration and assignation of all LLID are performed as described above, the transmission/reception parties of packets transmitted by the ONUs 110 or the OLT 100 in the EPON can be classified according to the LLID in the preamble.
FIG. 3 shows an EPON protocol specification in a draft v1.0. Referring to FIG. 3, an emulation function is integrated into an RS layer (Emulation Function layer) 208. According to the EPON specification, the RS layer 208 checks whether or not a destination of a packet is the RS layer, by means of the LLID. Herein, in general, a MAC layer 204 confirms whether or not a destination of a packet is the MAC layer itself by means of a destination address (DA) of the packet. In contrast, in the EPON, a packet filtering can be performed by a layer below the MAC layer 204.
Referring to FIG. 1, a party represented by the LLID varies according as a transmission direction of a packet is upward (ONU→OLT) or downward (OLT→ONU). In the case of upward transmission, an address represented by the LLID is a party which transmits the packet. In contrast, in the case of downward transmission, the address represented by the LLID is a party which receives the packet. That is, when the OLT 100 receives a packet from the ONUs 110, the OLT 100 compares a LLID in the packet with content of a LLID list registered to the OLT 100 and then determines whether the OLT 100 receives the packet or not. In contrast, when each of the ONUs 110 receives a packet from the OLT 100, each of the ONUs 110 checks whether the content of a LLID in the packet is equal to a LLID assigned to each of the ONUs 110 itself or not, and subsequently determines whether each of the ONUs 110 receives the packet or not.
Herein, when an attribute of a packet is a unicast, an ID value representing a destination is assigned to an ID field. In contrast, when the attribute of the packet is a broadcast, a default LLID for the broadcast is assigned. Accordingly, when each of the ONUs 110 receives a packet and a mode bit of the LLID in the packet represents a broadcast, then each of the ONUs 110 receives all packets. That is, when each of the respective ONUs 110 examines the mode bit of the LLID and the mode bit represents a unicast, each of the ONUs 110 either receives or does not receive a corresponding packet according to an ID. In contrast, when the mode bit of the LLID represents a broadcast, each of the ONUs 110-n receives a corresponding packet regardless of the ID.
However, only unicast and broadcast have been described with respect to the mode bit in the standardization, and a multicast mode, which can transmit data to receivers in a particular group, has not been accurately provided in the present IEEE 802.3ah draft.
Further, for a LLID registration, it is required that a registration process must pass through the present EPON specification. This process is mainly performed as an initialization process and then regularly (or irregularly, as the case may be) provides subsequent registration windows of opportunity to a party that requires to be registered.
However, in the case of a multicast group member, a particular group may be freely registered or terminated by means of a GARP multicast registration protocol (GMRP), which is a multicast group member registration protocol, and the registration or the termination must be performed regardless of a registration window of opportunity provided by an OLT. However, this can't be performed well through the conventional method for registration/termination of a LLID.